1. Technical Field
This invention relates to an indoor position determining system, particularly, although not necessarily exclusively, using ultrasound.
2. Background Information
There are many situations in which it is desirable to be able to determine the location of a moveable object, such as an item of equipment or a person, within a given area. It is known to provide room-level location using ultrasonic transmitters or tags attached to the objects or persons to be located; and a suitable ultrasonic receiver located in each room that can establish the presence or absence of a given tag. This can be useful for stock inventory or locating personnel but it is not accurate enough for some applications.
It has also been proposed to use an array of ultrasonic receivers distributed around a monitoring zone to determine the position of a tag within the zone using trilateration—i.e. using the time taken for the signal from the tag to reach different receivers. However this would not be very practical for a large installation as there would be a high equipment cost associated with a lot of separate receivers. Furthermore, the installation cost would also be high (for example, cabling for power and communication may need to be laid separately to each unit), and the end result can be unsightly if it is retro-fitted due to the need to have multiple units within each room.
Another problem with implementing this idea practically is that in real environments there are many surfaces, for example the walls of a room, from which the tag's signals can be reflected before being received. These can have the tendency to obscure the signals of interest and require complex processing to eliminate. For example a geometrical and a statistical approach for identifying and eliminating inaccurate distance measurements due to reflections are described in “A New Location Technique for the Active Office”, IEEE Personal Communications, Vol. 4, No. 5, October 1997, pp 42-47. The problems caused by reflections can be mitigated by increasing the number of receivers, but this just further increases the cost.
It has been proposed in WO 2006/013512 to use a single base station to determine the location of a tag using the line of sight signal from the tag as well as reflected signals caused by reflections off the walls, ceiling, floor and possible other surfaces in a room by use of a signature-matching method. A time-series signature of the signal and its reflections received by the base station is matched to stored model signatures or templates. Such an approach has a number of shortcomings: in particular, it provides poor positioning accuracy since signature-matching algorithms cannot perform well in noisy environments (such as when multiple received signals or their echoes overlap in time); furthermore, the accuracy of the method is limited to the number of candidate model signatures or templates, which is typically constrained by available processing power, memory space and time.